Speciated characterization of primary organic aerosol (POA) emissions from on road gasoline and diesel vehicles

Emissions from on road gasoline and diesel vehicles are major sources of primary organic aerosol (POA) to the atmosphere. Characterizing the chemical composition of this POA source has implications for the rate of secondary organic aerosol (SOA) formation and its resultant composition when these compounds evaporate during atmospheric dilution. Samples of vehicle exhaust particulates were collected in the Caldecott Tunnel (California highway 24) in July 2010 and analyzed by gas chromatography with high resolution time of flight mass spectrometry and vacuum ultraviolet ionization (GC/VUV-TOFMS). This 'soft' ionization technique allows for improved identification of hydrocarbons because they display a significant parent ion peak facilitating compound classification according to carbon number, degree of saturation and number of rings in their structure. The composition of POA was dominated by hydrocarbons with between 25 and 30 carbon atoms, consistent with being derived from motor oil with minimal influence from unburned diesel fuel and no observable influence from unburned gasoline. The dominant fraction of the mass was branched cyclic compounds with one, two and three rings and only minimal amounts of aromatics were observed. The calibrated sum of all speciated measurements agreed with the total measured organic carbon (85 ± 10 %) within analytical uncertainties indicating almost complete mass closure and an unprecedented level of speciation. These results are important for improving the representation of POA in models and have implications for the formation rate and product composition of secondary organic aerosol due to the different atmospheric oxidation trajectories of the identified functional groups.